v2.4.0.6
Document and Entity Information (USD $)
In Billions, except Share data, unless otherwise specified
3 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2012
Jan. 31, 2013
Jun. 30, 2012
Entity Information [Line Items]        
Document Type 10-K      
Amendment Flag false      
Document Period End Date Dec. 31, 2012      
Document Fiscal Year Focus 2012      
Document Fiscal Period Focus Q4      
Entity Registrant Name American Capital Agency Corp      
Entity Central Index Key 0001423689      
Entity Filer Category Large Accelerated Filer      
Current Fiscal Year End Date --12-31      
Entity Common Stock, Shares Outstanding     338,936,470  
Trading Symbol AGNC      
Entity Well-known Seasoned Issuer   Yes    
Entity Voluntary Filers   No    
Entity Current Reporting Status   Yes    
Entity Public Float       $ 10.2